Off the top of my head, there only has to be 4 % hydrogen gas present for there to be a possible explosion if oxygen is also present. They actually were surprised to find hydrogen at about 3 % when they looked for the first time into sections of reactor #2. Their main plan of attack, from what the news report state, is to insert nitrogen which is heavier then hydrogen. So they must be forcing the hydrogen up and out somewhere. Probably those two flame-offs that could be seen on a Fukushima web cam last night. Concerning passive catalytic hydrogen recombiner, do not personally know, but below is a link to an entire book on the subject.
Passive catalytic hydrogen recombiners for nuclear power stations